FOA CFOS/A flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Critical angle

The minimum incidence angle at the core-cladding boundary for total internal reflection, equal to arcsine(n_cladding divided by n_core). Below it, light refracts out of the core.

Numerical aperture

A dimensionless measure of a fiber's light-gathering ability, equal to the square root of (n_core squared minus n_cladding squared). Its arcsine gives the acceptance angle in air.

Attenuation formula

Loss in decibels equals ten times the base-10 logarithm of input power divided by output power. Divide by length in kilometers to express loss per kilometer.

Rayleigh scattering

Intrinsic loss from microscopic density fluctuations in the glass. It varies roughly as one over wavelength to the fourth power, which is why 1550 nm loss is far below 850 nm loss.

Modal dispersion

Pulse spreading in multimode fiber because different modes travel different path lengths and arrive at different times. It limits bandwidth-length product and is absent in true single-mode fiber.

Single-mode cutoff wavelength

The wavelength above which only the fundamental mode propagates. It must be below the operating wavelength, typically under 1260 nm for standard single-mode fiber used at 1310 nm.

Macrobend loss

Loss caused by bending fiber below its critical radius. It increases with wavelength, so long-wavelength testing reveals bends that shorter wavelengths may not.

Return loss

A positive decibel value describing reflected power relative to incident power at a component. Higher return loss means a better, less reflective connection.

OM5 wideband multimode

Laser-optimized 50/125 micrometer multimode fiber designed for short-wavelength division multiplexing, allowing multiple channels between roughly 850 and 953 nm over one fiber.

OS2 single-mode fiber

Low-water-peak single-mode fiber usable across roughly 1260 to 1625 nm, including the E-band. It supports CWDM and DWDM in metropolitan and access networks.

G.657 bend-insensitive fiber

Single-mode fiber specified for reduced macrobend loss at small radii, used in indoor, drop, and patch-cord applications where tight routing is unavoidable.

Chromatic dispersion

Pulse spreading because different wavelengths travel at different speeds in glass. In standard single-mode fiber it is near zero at 1310 nm and about 17 ps/nm-km at 1550 nm.

Polarization mode dispersion

Differential delay between the two polarization states caused by core asymmetry and stress. It is statistical, so budgets use a coefficient in picoseconds per root kilometer.

CWDM channel spacing

Coarse wavelength division multiplexing uses 20 nm spacing across roughly 1270 to 1610 nm. It tolerates uncooled lasers and cheaper filters than dense WDM.

DWDM channel spacing

Dense wavelength division multiplexing uses 50 or 100 GHz spacing near 1550 nm, requiring tightly controlled lasers and narrow filters to keep channels separated.

Optical link budget

The allowed loss between transmitter and receiver, equal to launch power minus receiver sensitivity, minus a design margin, compared against total fiber, splice, and connector loss.

Graded-index multimode fiber

Fiber whose core refractive index decreases smoothly outward, continuously refracting rays and reducing modal dispersion compared with step-index multimode fiber.

Horizontal cabling limit

Under TIA-568 the permanent horizontal link is limited to 90 meters, with up to 10 meters of patch cords shared between the work area and telecommunications room.

Cable bend radius rule

Minimum bend radius is usually ten times the cable outer diameter under installation load and twenty times in the installed, unloaded state, unless the manufacturer specifies otherwise.

MPO polarity methods

Method A uses straight key-up-to-key-down trunks, Method B uses reversed fiber pairs, and Method C uses crossed pairs. The method must match the duplex patch cords used.

APC connector end face

An eight-degree angled, typically green connector that reflects light away from the source, giving much higher return loss than flat UPC connectors. Never mate APC to UPC.

Plenum cable rating

OFNP or OFCP cable is required in air-handling spaces under NEC Article 770 because its jacket emits low smoke and flame in a fire.

Multimode fiber color codes

OM1 is beige, OM2 is black, OM3 and OM4 are aqua, and OM5 is lime green. Single-mode UPC is blue and APC is green.

Maximum installation tensile load

The pulling force a cable may see during installation, set by the manufacturer and often near 220 newtons for indoor tight-buffered cable. Exceeding it stretches or breaks fibers.

Fusion splice acceptance loss

Field acceptance for a fusion splice is commonly 0.1 dB or less, while field-terminated connectors are typically accepted at 0.3 dB or less, per project specification.

Tier 1 versus Tier 2 testing

Tier 1 uses an optical loss test set for insertion loss and length. Tier 2 adds an OTDR trace to locate and characterize splices, connectors, and faults along the span.

Encircled flux launch condition

A standardized multimode launch condition that limits how much power enters outer modes. Without it, insertion loss readings vary widely between test setups.

OTDR event dead zone

The minimum distance after a reflective event before another event can be detected. It shortens with shorter pulse widths, which also reduce dynamic range.

OTDR attenuation dead zone

The distance after a reflective event over which backscatter cannot be measured accurately, longer than the event dead zone and also set by pulse width.

Reflective versus non-reflective events

Connectors show as reflective spikes on an OTDR trace, while fusion splices and bends appear as non-reflective step losses. This distinction guides fault isolation.

End-face inspection and cleaning

Contamination is the leading cause of connector failure. Inspect with a scope and clean with lint-free wipes and approved solvent before every mating.

10GBASE-LR reach

A 1310 nm single-mode Ethernet standard reaching about 10 kilometers over OS2 fiber, commonly used for building and campus backbone links.